You will get that when you are logged on to AOL and ichat at the same time under the same name.

However I have been getting lots of AIM porn spam and it's pissing me off. It's as bad as my email...which really isn't that bad compared to some. But its annoying as hell to come home at the end of the day and find 4 messages. I'm thinking about blocking all none buddies.

I doubt you can block AOL system messages, as they come through the network itself and not from a "mere" user. Ignore as usual.

As for the porn solicitations, I get those too, quite often. I think it started after I entered a chat room a while back. I just choose not to accept messages from screen names with girls' names and lots of numbers.

That aolsystemmsg is a new thing. You can now be logged in to AIM from multiple locations. It used to be that if you were logged in already then logged in again from somewhere else, your first login would get booted.
Now you can be logged in more than once. If one login is "away" and the other is connected, all messages go to the connected login. If both are connected, or both are away, all messages go to both.